The present invention generally relates to gauges, and more particularly, to prosthetic gauges used for evaluating the fit of a prosthetic implant prior to operative reconstruction of a joint.
Orthopaedic surgeons frequently find it nesessary to utilize a gauge to select the proper size of prosthetic implant for replacement of a joint surface. Gauges can be adapted for use to select the proper size of prosthesis for many types of joint surfaces, such as the femoral head, the acetabulum, and the tibial surface of the knee. In the case of an endoprosthesis where an artificial ball joint surface is utilized to articulate against the natural acetabular cartilage, determination of the proper ball size to mate with the acetabulum is particularly important.
A variety of techniques have been used to select the proper size of endoprosthesis for a hemiarthroplasty. These current methods are often very subjective and can be misleading. Furthermore, these known techniques generally do not provide a means for assessing the contact areas between the prosthetic replacement and the acetabular articular cartilage.
The necessity and importance of proper prosthetic selection and the concept of optimal fit for an endoprosthesis is supported in Chapter 5, "Pressure Distribution in the Hip and Selection of Hemiarthroplasty," by W. H. Harris, P. D. Rushfeldt, C. E. Carlson, J. M. Scholler, and R. W. Mann in THE HIP, Proceedings of the Third Open Scientific Meeting of the Hip Society, 1975, pp. 93-98. This article indicates that poor fit of the ball of the endoprosthesis to the articular cartilage may lead to failure of the endoprosthesis. It indicates that "mismatches in either direction, that is, a prosthesis too large or too small for the socket, could make the local load on the acetabular cartilage higher, possibly even excessively high for cartilage survival." The authors indicate that proper prosthetic selection could improve the results from the use of endoprostheses and possibly improve the longevity of these implants.
The article also describes many of the techniques known for selecting the endoprosthesis size. One technique described as a method of size selection is the "shuck" test. The "shuck" test presumably means that a series of prostheses are tried in the acetabular socket, and the one with least "shuck" is selected. This method is very crude and subjective.
This same article states that "current clinical practice also involves some measurement with calipers, but at times these measurements may be inappropriate, inaccurate or even misleading." Calipers only provide a measurement in one arc and not the whole surface profile. The article also notes that the determination of prosthesis fit is frequently made on the basis of a feeling of suction between the socket and the prosthesis. This feeling of suction has been shown to have almost nothing to do with the fit of an endoprosthesis.
This article also illustrates and describes the use of transparent glass spheres which were used to study contact area between sphere and acetabular cartilage by direct visualization. The article indicates that the crux of the matter is to be able to determine the contact surface, that is, the area over which the prosthesis makes good contact with the socket. The transparent spheres were used so that one could see the contact profile by direct observation. However, the disadvantage of this technique is that all of the cartilage surface can be seen through the transparent spheres. Consequently, it is difficult to determine whether the cartilage surface is actually in contact with the sphere or just below the surface of the sphere, but still visible. It is also noted that glass spheres would not be desirable for use in an operative procedure for safety reasons, such as potential breakage.
A "go or no go" femoral head gauge is also disclosed in this same article. This template-type gauge is used to determine the diameter of the femoral head. The correct size of prosthesis to use is one whose diameter equals the smallest hole through which the femoral head will pass. However, the measurements obtained by the "go no go" gauge template provide no information about contact at the zenith of the acetabulum.
Proper contact between a prosthetic implant and its mating bone surface is also important when using prostheses which are adapted for bony ingrowth. Typically, prostheses adapted for bony ingrowth utilize a porous material on the prosthesis surface which will be in contact with the bone surface. Such bone ingrowth prostheses do not utilize bone cement on the bone ingrowth surface. This enables the bone to be in contact with this porous surface, so as to enable the bone to grow into the porous surface. For successful bone ingrowth, it is important that the bone be in good direct contact with the porous surface of the prosthesis. Thus, it is very important that there is proper fit and good contact area between the implant and its mating bone surface to achieve the best fit. One such porous material which is suitable for bony ingrowth is disclosed in U.S. Pat. No. 3,906,550 to Rostoker and Galante.